Dicamba and 2,4-D Drift Experiment

Horseradish plant chlorosis from dicamba treatments developed over the first 2 wk and then progressively decreased until the end of the growing season (Table 2). Although chlorosis data could be pooled over years for 7, 14, 28, and 42 DAT, late-season chlorosis at 56 DAT was more prominent in 2015 than in 2016. Dicamba applied at simulated drift rates did not cause chlorosis in horseradish. Dicamba applied at 1/2X, 1X, and 2X rates caused 4%, 7%, and 13% chlorosis, respectively, at 7 DAT, and 5%, 11%, and 23% chlorosis, respectively, at 14 DAT. However, at 28 DAT, chlorosis was just 9% with dicamba at the 2X rate and not significant at the 1/2X or 1X rates. Chlorosis at 56 DAT was slightly higher in 2015 than 2016, with 4%, 2%, and 3% chlorosis at the 1/2X, 1X, and 2X rates, respectively, during 2015, whereas in 2016 there was no significant chlorosis.

Table 2 Effect of dicamba and 2,4-D on horseradish chlorosis and necrosis in drift experiments at Edwardsville, IL, in 2015 and Medora, IL, in 2016. ^a

^a Abbreviation: DAT, days after treatment.

^b Means separated using arcsine transformation.

^c Nontransformed means separation presented.

^d Data pooled over years for chlorosis at 7, 14, 28, and 42 DAT, and necrosis at 14, 28, 42, and 56 DAT.

^e Effects of dicamba and 2,4-D were analyzed as a one-way ANOVA for all combinations of herbicide and rate, with means separated using Fisher’s protected LSD (P≤0.05).

Generally, horseradish plant injury was more severe when 2,4-D was applied compared with dicamba. At the 7 and 14 DAT evaluations, chlorosis with 2,4-D occurred primarily at or above the 1/2X rate, whereas at or beyond 28 DAT, some chlorosis was observed with 2,4-D at or below the 1/100X rate. When rates of 1/2X or higher were compared, 2,4-D caused 17% to 22% chlorosis at 7 DAT and 9% to 20% chlorosis at 14 DAT. Chlorosis was observed to be lower with 2,4-D at the 2X rate because of the rapid progression from chlorosis to necrosis. Although Marple et al. (Reference Mohseni-Moghadam, Wolfe, Dami and Doohan2008) indicated that both 2,4-D and dicamba at 1.4 to 2.8 g ae ha⁻¹ caused chlorosis of cotton (Gossypium hirsutum L.) leaves, similar rates did not cause chlorosis of horseradish leaves in this study. This indicates that horseradish is less likely to exhibit symptoms of 2,4-D and dicamba chlorosis injury due to drift compared with other susceptible crops such as bell pepper (Capsicum annuum L.), cotton, broccoli (Brassica oleracea L. var. botrytis), peanut (Arachis hypogaea L.), and soybean (Andersen et al. Reference Andersen, Clay, Wrage and Matthees2004; Johnson et al. Reference Lange, Fischer, Humphrey, Seyman, Baghott, Sumner, Krantz, Broadbent, Williams and Baghott2012; Marple et al. Reference Mohseni-Moghadam, Wolfe, Dami and Doohan2008; Mohseni-Moghadam and Doohan 2015).

Although an interaction (P≤0.05) was observed between year and herbicide treatment for horseradish necrosis at 7 DAT, necrosis data were pooled over years for 14, 28, 42, and 56 DAT due to no observed interactions (P≥0.05). The foliar necrosis caused by dicamba was slow to develop, reached a maximum level at 28 DAT, and then progressively decreased over time. Moreover, significant amounts of necrosis were observed only with the 1X and 2X rates of dicamba, and rates at or below the 1/100X did not cause necrosis. At 7 DAT, dicamba at the 1X and 2X rates caused only minor injury, with 5% and 7% necrosis in 2015 and only 2% and 3% necrosis in 2016, respectively. At 14 and 28 DAT, dicamba at the 1X and 2X rates caused 5% and 18%, and 18% and 58% necrosis, respectively. The injury from dicamba decreased over time and was only 27% at 56 DAT for the 2X application rate. In comparison, necrosis from 2,4-D treatments continued to increase over time. At 7 DAT in 2015, 2,4-D at the 1/2X, 1X, and 2X rates caused 17%, 43%, and 72% necrosis, respectively, while in 2016, 2,4-D at the same rates caused 2%, 8%, and 7% necrosis, respectively. The difference in necrosis between years may be related to cumulative rainfall before and after herbicide application in 2015 and 2016. Total rainfall was 66.3 and 65.6 cm in 2015 and 2016, respectively. In 2015, 32.3 cm was received from the date horseradish was planted to the date of herbicide application, compared with 19.3 cm in 2016. In 2015, 5.8 cm of rainfall was received over a 7-d period after application compared with only 0.9 cm of rainfall for the same period in 2016; thus, plants may have been growing more actively in 2015 compared with 2016, resulting in more injury in 2015. The average high temperature (31 C) for 7 DAT was similar for both years. Foliar necrosis also developed later in 2016 than in 2015. When data were pooled over years, 2,4-D at the 1/2X rate or greater increased necrosis progressively from 25% to 73% at 14 DAT, reaching 90% to 98% necrosis at 42 DAT. Application of 2,4-D at or above the 1/2X rate caused 98% to 99% foliar necrosis on all horseradish plants at 56 DAT. Necrosis resulting from drift rates of 2,4-D was not significantly different from the zero necrosis caused by dicamba, except at 42 and 56 DAT for the 1/1,000X rate, at 14% and 17%, respectively.

Horseradish plant stands were also affected by the two PGR herbicides, although reductions in plants per hectare were greater following 2,4-D applications than dicamba applications (Table 3). Dicamba at the 2X rate reduced plant population by 14% and 28% at 42 and 56 DAT, respectively. At 28 DAT, 2,4-D at the 1X and 2X rates reduced plants per hectare by 79% and 96%, respectively. At 42 DAT, reductions in plant populations were found for application rates of 1/2X, 1X, and 2X. Reductions in plant populations of 94%, 99%, and 100% occurred at 56 DAT when 2,4-D was applied at the 1/2X, 1X, and 2X rates, respectively (Table 3).

Table 3 Effect of dicamba and 2,4-D on horseradish plants per hectare and plant height in drift experiment pooled over 2015 and 2016 growing seasons. ^a

^a Abbreviation: DAT, days after treatment.

^b Effects of dicamba and 2,4-D were analyzed as a one-way ANOVA for all combinations of herbicide and rate, with means separated using Fisher’s protected LSD (P≤0.05).

Horseradish plant height was reduced more following 2,4-D applications compared with dicamba applications (Table 3). At 14 DAT, dicamba at the 1/2X, 1X, and 2X rates reduced horseradish plant height from 52 cm, in the nontreated control, to between 37 and 42 cm. The 2X rate of dicamba reduced height by 65%, 74%, and 72% at 28, 42, and 56 DAT, respectively, compared with the nontreated control; 2,4-D at the 1/2X, 1X, and 2X rates also reduced horseradish plant heights at all measurement dates. However, unlike dicamba, 2,4-D reduced horseradish heights by 20% when applied at the 1/100X drift rate at 56 DAT. By 42 DAT, 2,4-D at the 1X and 2X rates had reduced horseradish plant heights to 0 cm, and no regrowth was observed by 56 DAT; complete plant death had occurred for 2,4-D with rates greater than 1/2X at 56 DAT. An increase in height was observed at 14 DAT at the two lowest 2,4-D drift rates (increase of 8 to 10 cm), suggesting an early hormetic effect, but this did not persist beyond the first rating. Smith et al. (Reference Walters and Wahle2017) also found that dicamba at 19 to 37 g ae ha⁻¹ caused no cotton height reduction, whereas the same rates of 2,4-D caused 17% cotton height reduction, suggesting that 2,4-D can cause greater amounts of plant height reductions to certain crops compared with dicamba, depending on crop sensitivity.

Dicamba and 2,4-D affected horseradish yield differently (Table 4). Dicamba only reduced horseradish yield, compared with the nontreated, when applied at the 1X and 2X rates; 2,4-D reduced yield when applied at rates of 1/1,000X or greater. Dicamba reduced yield by 35% and 69% at the 1X and 2X rates, respectively. In comparison, a 41% to 100% yield loss occurred when 2,4-D was applied at rates of 1/1,000X or more. Schroeder et al. (1983) also noted greater yield loss in sugarbeet (Beta vulgaris L.) from 2,4-D applications than from dicamba applications when these herbicides were applied at similar rates and timings. However, snap bean (Phaseolus vulgaris L.) injury and yield reductions were greater from dicamba than from 2,4-D (Colquhoun et al. Reference Colquhoun, Daniel and Tittmeyer2014).

Table 4 Effect of dicamba and 2,4-D on horseradish yield and primary root weight, length, and diameter in drift experiments pooled over the 2015 and 2016 growing seasons.

^a Effects of dicamba and 2,4-D were analyzed as a one-way ANOVA for all combinations of herbicide and rate, with means separated using Fisher’s protected LSD (P≤0.05).

Horseradish primary root characters (weight, length, and diameter) were impacted by multiple rates of both herbicides (Table 4). Although dicamba at the 1/2X and 2X rates reduced primary horseradish root weight, dicamba at the 2X rate also reduced both primary root length and diameter. Reductions in primary root weight and diameter occurred when 2,4-D was applied at the 1/1,000X rate or higher. Set roots were also influenced by the application of dicamba and 2,4-D (Table 5). In 2015, dicamba at all rates reduced the number of set roots produced per large primary root. Dicamba at all rates except 1/1,000X reduced set weight, although no dicamba rates reduced average set diameter. In 2016, only the 2X rate of dicamba reduced set number and weight. In 2015, 2,4-D reduced the number of sets per primary root and set weight regardless of rate (except for set weight at the 1/10,000X rate). In 2016, 2,4-D rates at 1/2X and above reduced set number, while rates at 1/100X and above reduced set weight. Differences between years may be related to the rainfall differential mentioned previously; greater rainfall occurring early in the season may have increased plant growth rate in 2015 compared with 2016, resulting in increased herbicide sensitivity.

Table 5 Effect of dicamba and 2,4-D rates on horseradish secondary roots (sets) in drift experiment separated by the 2015 and 2016 growing seasons. ^a

^a Effects of dicamba and 2,4-D were analyzed as a one-way ANOVA for all combinations of herbicide and rate, with means separated using Fisher’s protected LSD (P≤0.05).

^b No plants remained at harvest in the 1X and 2X treatments, and only one plant remained in the 1/2X treatment; therefore, no set data was collected.

The residue analysis of processed horseradish roots was pooled over years (Table 6). Only roots from plants sprayed with dicamba at the 2X rate had greater dicamba residues than the nontreated control. The residue level in these roots was 0.32 parts per billion (ppb), which is lower than the EPA tolerance of 100 ppb in root crops (USEPA 2018). Compared with the nontreated control, applications of 2,4-D did not affect the root residue levels at the drift rates evaluated. No residue data could be collected for the 1X and 2X rates of 2,4-D, because there was no harvestable tissue at these rates. Only one plant survived the 1/2X rate and was analyzed for residue, but it was not included in the statistical analysis. Although 2,4-D was not applied to the control plots during the study, 2,4-D was detected in the control plants. There were also much greater values of residue detected for 2,4-D than dicamba. There was no baseline testing of the sets used to initiate this study, and it is therefore unknown whether 2,4-D was present in horseradish sets at planting. However, in both years, the sets used to plant the study were taken from a field that had an early burndown application of 2,4-D and glyphosate before horseradish planting. This is a common burndown practice in horseradish production and may have resulted in contamination of the parent stock. While the 2,4-D residue levels detected were well below the EPA tolerance limits, more research is needed to determine 2,4-D accumulation rates and changes in residue levels in horseradish tissue over time.

Table 6 Horseradish root residue analysis of dicamba and 2,4-D at harvest in drift experiments pooled over the 2015 and 2016 growing seasons.

^a Abbreviation: ppb, parts per billion.

^b No plants remained in the plot at harvest.

^c Only one plant remained to sample; therefore, residue amount (ppb) was not analyzed statistically.

Regression was used to determine the relationship between early-season horseradish plant injury (visual height reduction and foliar necrosis) caused by dicamba and resulting yield (Figure 1). A relatively low amount of horseradish plant visual height reduction of 15%, caused by dicamba at 7 DAT, would predictably reduce root yield from about 19.2 to 13.2 metric tons ha⁻¹, a 31% yield loss (Figure 1A). Following 2,4-D injury, yield was reduced from 13.2 to 2.3 metric tons ha⁻¹ (82% yield loss) at 15% injury (unpublished data). Distribution of 2,4-D height reduction data was divided into two categories: the first with no height reduction and no yield loss, with variable yields ranging from 5 to 10 tons ha⁻¹; and the second with 15% or more visual height reduction and almost no yield at 0 to 2 tons ha⁻¹. There were no observations at 5% or 10% injury. Given this categorical distribution, regression analysis of 2,4-D drift rate effects on yield as a function of visual height reduction did not produce a descriptive model. A regression equation was not reported for 2,4-D drift rate effects on yield as a function of foliar necrosis for the same reason. At a low level of foliar necrosis injury of 10%, yield loss from dicamba would reduce yield from 18.7 to 7.8 metric tons ha⁻¹, a 58% yield loss (Figure 1B), whereas for 2,4-D at this injury level, the yield would be reduced from 13.3 to 5.0 metric tons ha⁻¹, a 62% yield loss (unpublished data). Similar to plant height reduction, greater amounts of horseradish yield loss were caused by 2,4-D compared with dicamba at equivalent low amounts of foliar necrosis. These results indicate that horseradish is more sensitive to yield loss from 2,4-D applications compared with dicamba applications.

Figure 1 (A) Horseradish yield (metric tons ha⁻¹) as a function of visual height reduction (%) at 7 DAT as a result of dicamba simulated drift: y=19.16 − 0.3993x, R²=0.65, P=0.0001. (B) Horseradish yield (metric tons ha⁻¹) as a function of foliar necrosis (%) at 14 DAT as a result of dicamba simulated drift: y=19.16 – 0.3993x, R²=0.65, P=0.0001.

Simulated drift rates of dicamba result in less horseradish injury and yield loss than 2,4-D. The dicamba drift rates evaluated (1/10,000X, 1/1,000X, and 1/100X) caused little to no horseradish plant injury, which was then later reflected in the absence of any associated yield reductions. In contrast, recent research has indicated that dicamba drift can result in significant specialty crop yield loss, particularly bell pepper and broccoli (Mohseni-Moghadam and Doohan 2015), as well as tomato (Solanum lycopersicum L.) (Kruger et al. Reference Mark and Wyse2012). Furthermore, simulated drift on dry edible bean (Phaseolus vulgaris L.), dry edible pea (Pisum sativum L.), and potato (Solanum tuberosum L.) indicated differences between these crops in sensitivity to sublethal doses of dicamba (Hatterman-Valenti et al. Reference Hatterman-Valenti, Endres, Jenks, Ostlie, Reinhardt, Robinson, Stenger and Zollinger2017). Although similar 2,4-D drift rates caused minimal late-season chlorosis, necrosis, and visual height reduction in horseradish, significant yield reductions were observed for this herbicide. Growers should be aware that even minimal horseradish plant injury with 2,4-D would most likely result in yield loss, which would not be the case with dicamba drifting onto horseradish plants.

Horseradish was more sensitive to drift exposure and standard application rates of 2,4-D compared with dicamba. While drift rates of dicamba may not cause horseradish injury or yield loss, 2,4-D at similar rates will most likely cause injury and yield loss if the rate drifted is 1/1,000X or greater. However, the development of injury symptoms on horseradish may vary depending on herbicide rates used and environmental factors at the time of application. Regression models may not always accurately determine horseradish yield loss from early-season height reduction or foliar necrosis injury due to this variability. Additionally, significant injury may not be detectable early in the growing season to determine whether a field replant situation may be necessary to avoid significant yield loss. Although horseradish may potentially be at a higher risk for injury and yield loss from 2,4-D drift, growers should consider the use of 2,4-D–resistant crops in rotation with horseradish to gain control of troublesome volunteer horseradish. Both Johanning et al. (Reference Kruger, Johnson, Doohan and Weller2016) and Rundle et al. (2007) indicated that 2,4-D is highly effective for volunteer horseradish control. Although 2,4-D can be a tool to manage this pest, the management of volunteer horseradish is still a challenge when horseradish is grown in a relatively short rotation cycle with other crops such as corn and soybean (Jenkins et al. Reference Jenkins, Krausz, Matthews, Gage and Walters2017). Our results should help growers better understand the importance of specialty crop field placement in areas where 2,4-D- and/or dicamba-resistant crops are prevalent, as well as how to better use these crops in relation to possible drift events.

Dicamba Herbicide Carryover Experiment

Because only imperceptible differences were observed with all soybean and horseradish parameters evaluated, means are presented across herbicide treatments (Table 7). Dicamba application did not result in any difference in soybean population or soybean growth stage, regardless of application timing or rate, in either 2014 or 2015 (Table 7). Most importantly, no soybean injury was observed following application of dicamba or dicamba plus glyphosate during either 2014 or 2015 (unpublished data).

Table 7 Influence of dicamba on dicamba-tolerant soybean plant population and growth stage in 2014 and 2015 and on subsequent year’s horseradish emergence, plant height, plant population, and yield pooled over the 2015 and 2016 growing seasons. ^a

^a Abbreviations: DAP, days after planting; WAP, weeks after planting; EPOST, early postemergence, V2 soybean; MPOST, mid-postemergence, V4 soybean; PRE, preemergence, within 2 days of planting and before emergence of crop.

^b MON 76832, 120 g ae L⁻¹ dicamba plus 242 g ae L⁻¹ glyphosate premix; MON 119096, 350 g ae L⁻¹ dicamba alone.

^c Growth stage is measured at 60 DAP.

Dicamba carryover did not negatively affect horseradish (Table 7). Horseradish emergence occurred approximately 23 DAP and was not affected by either dicamba or dicamba plus glyphosate applications. No injury to horseradish was observed due to carryover from dicamba or dicamba plus glyphosate applied in soybean the previous year (unpublished data). Horseradish plant heights were not reduced when comparing dicamba or dicamba plus glyphosate application treatments at 4, 6, and 8 WAP with the nontreated control. Stand counts of emerged horseradish plants also did not differ at 4 and 8 WAP. Horseradish yields in dicamba carryover treatments averaged 18.4 metric tons ha⁻¹ (98% of the nontreated control) and were all similar to the nontreated control (Table 8). There were also no differences between herbicide carryover treatments for the horseradish root measurements collected.

Table 8 Effect of dicamba carryover when applied in a dicamba-tolerant soybean system on subsequent year’s horseradish yield, primary root length, diameter, and weight and secondary roots (sets) pooled over the 2015 and 2016 growing seasons. ^a

^a Abbreviations: EPOST, early postemergence, V2 soybean; MPOST, mid-postemergence, V4 soybean; PRE, preemergence, within 2 days of planting and before emergence of crop.

^b MON 76832, 120 g ae L⁻¹ dicamba plus 242 g ae L⁻¹ glyphosate premix; MON 119096, 350 g ae L⁻¹ dicamba alone.

Horseradish injury or height or yield reduction was not observed following any herbicide carryover treatment, including those in which dicamba was applied at rates to mimic an overapplication. This suggests that the use of dicamba in-season on dicamba-resistant soybean will be safe when rotating a field the following year to horseradish. However, it is important that growers using this technology are aware of the injury risk to specialty crops grown in close proximity when using PGR herbicides, and they should know that potential for injury is dependent upon the herbicide used, application rate, and timing of exposure, as well as prevailing weather conditions.

Although both dicamba and 2,4-D are PGR herbicides that can cause injury to many annual vegetable and perennial fruit crops, they have great potential when used with recently developed soybean technologies (e.g., XtendiMax^™ and Enlist^TM systems) to control may difficult to control broadleaf weeds, especially those that are GR. While these tools may help growers increase crop production, the potential for off-target movement should remain a primary consideration. Results of this trial indicated differences in horseradish response to dicamba and 2,4-D drift, similar to many other annual broadleaf crops such as bell pepper, cotton, broccoli, peanut, and soybean (Andersen et al. Reference Andersen, Clay, Wrage and Matthees2004; Johnson et al. Reference Lange, Fischer, Humphrey, Seyman, Baghott, Sumner, Krantz, Broadbent, Williams and Baghott2012; Marple et al. Reference Mohseni-Moghadam, Wolfe, Dami and Doohan2008; Mohseni-Moghadam and Doohan 2015). Although the two herbicides have a similar mode of action, they are in different chemical families and affect horseradish differently. Simulated drift rates of dicamba were shown to not cause horseradish injury, whereas 2,4-D drift rates can cause substantial late-season injury and yield loss. Both cotton (Everitt and Keeling Reference Everitt and Keeling2009) and sugarbeet (Schroeder et al. 1983) have been reported to respond similarly to simulated drift rates, with dicamba producing less injury and yield loss than 2,4-D.

Our results provide additional evidence that PGR-resistant soybean growers should monitor the locations of specialty crops on the landscape. Drift risks and yield loss may be minimized by avoiding planting horseradish and PGR-resistant soybean in adjacent fields, being aware of weather conditions that can potentially increase the chance of a drift event, using only properly labeled herbicides, and thoroughly cleaning sprayers before spraying other pesticides on or near a sensitive crop.